In science class we are currently learning about elements, the periodic table etc. Today we all had our own element where we had to draw the valance electrons of it(in the shells) and then create a formula for it when it mixes with other elements. My element was Iodine, a non-metal:

You can see that it has 7 valance electrons in the 5th shell, so when it meets with a metal, it will gain 1 valance electron so it can become like its noble gas.

When it met with K:

Since K only had 1 valance electron, it was trying to get rid of it so it could become a noble gas, and since I needed to gain 1, they attracted.

Next was NA, these 2 attracted as well since NA needs to loose one electron and I gain. 

The last element Iodine bonded with was Li(lithium) 

Here is a table to demonstrate the formulas:

As you can see,  the elements that give away the electrons(cations) are now positively charged and the anions(the atoms that gained electrons) are now negative but have a complete valance shell.

 

So after all, atoms attract when they are positive/negative and when they can complete their valance shells.